Partial Discharge and Insulation Failure

PARTIAL DISCHARGE & INSUALTION FAILURE Dr Colin Smith IPEC Ltd. 2005

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INTRODUCTION

Partial Discharge (PD) is an electrical discharge that does not completely bridge the space between two conducting electrodes. The discharge may be in a gas filled void in a solid insulating material, in a gas bubble in a liquid insulator or around an electrode in a gas. When partial discharge occurs in a gas, it is usually known as corona. Partial discharge is generally accepted as the predominate cause of long term degradation and eventual failure of electrical insulation. As a result, its measurement is standard as part of the factory testing of most types of high voltage equipment. In addition, partial discharge activity is very often monitored on in-service equipment to warn against pending insulation failure. Test specifications set a maximum permissible level for partial discharges depending on the type of equipment being tested and the insulating material used. The principle behind such a specification is that discharges below a certain size cause minimal damage to the insulation. As insulation systems have increasingly moved towards polymers, acceptable discharge levels have lowered dramatically as they are less resistant to damage by discharge. This section will look at the physics behind the phenomenon of partial discharge, the effects partial discharge has on insulating systems and the failure mechanisms these can lead to. 2.0

HISTORY OF DISCHARGE MONITORING

Partial discharge has been observed as a phenomenon occurring in stressed high voltage insulation since the turn of the century. It became of increasing academic interest from the 1930s when its degrading effect on high voltage insulation became increasingly problematic. Early studies used ultrasonic detection 1 techniques to assess discharge activity in oil . In the 1950's theoretical and practical studies led by John Mason looked at how discharge activity could lead to previously unheard of breakdown processes like 2-4 electrical treeing . From the 1960s to the present time, partial discharge has been studied intensively in terms of the fundamental physics behind it, its effect on insulating systems and how best it can be measured and monitored with time.

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DISCHARGE PROCESS IN VOIDS

Solid insulators are manufactured to give an even distribution of electrical stress between the conducting electrodes. However, in practice this is virtually impossible to achieve. Manufacturing processes invariably give rise to small cavities or voids in the insulation bulk. These cavities are usually filled with a gas of lower breakdown strength than the surrounding solid. In addition to this the permittivity of the gas is invariably lower than that of the solid insulation, causing the field intensity in the cavity to be higher than that in the dielectric. Therefore under the normal working stress of the insulation, the voltage across the cavity may exceed the breakdown value and initiate electrical breakdown in the void.

Figure 1, Equivalent circuit for cavity in insulator Assume a solid insulator of thickness d contains a disc shaped cavity of thickness t and area A, as shown in Figure 1. In the equivalent circuit the capacitance Cc corresponds to the cavity, Cb corresponds to the capacity of the dielectric that is in series with Cc and Ca is the capacitance of the rest of the dielectric. 2

Given that capacitance C, in Farads/m , is given by;

C= Where;

ε0εr A d -12

ε0 = permittivity of free space = 8.854 x 10 εr = relative permittivity A = area between electrodes d = separation of electrodes

Fm

-1

If we assume that the gas in the cavity (of thickness t) in figure 1 has a relative permittivity of approximately 1, then:

Cc =

ε0A t

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and,

Cb = where:

ε0εr A d−t

εr = relative permittivity of the solid insulator

As Cb and Cc essentially form a capacitive divider, the voltage across the cavity, Vc, can be expressed as;

Vc =

Cb Va Cc + C b

Substituting into the above equation gives;

Vc =

Va 1 d 1+ −1 εr t

Therefore electrical field strength across the cavity (Ec) is given by the equation,

d

Ec = Ea t 1+

1 d −1 εr t

Given that in most circumstances t